Due to its lubricating mechanism, a two-cycle engine releases unburnt lubricating oil together with exhaust gas, which causes smoke.
From the viewpoint of environmental pollution, it has been desired to reduce smoke as much as possible. In Japan, there has already been extensively marketed low-smoke types of lubricating oil for two-cycle engines whose base oil contains polybutene or polyisobutylene as a major component that generates smoke less than mineral oils.
However, since polybutene accelerates friction between a piston and a cylinder due to its viscosity(stickiness), it inevitably reduces engine power more than mineral oils. Furthermore, starting becomes hard or troublesome when such a type of oil is applied in motors whose capacity is too small to start with a starter motor, or recoil types of engine such as a lawn-mower engine started by winding and then pulling a string around a crank pulley, a chain saw or a generator.
Polybutene has been considered to cause clogging with carbon in an exhaust system less frequently than mineral oils. It, however, has been reported that polybutene produces more emulsion sludge at low temperature than mineral oils when used in particular two-cycle engines, for example, an engine of a motorcycle used for newspaper delivery which is exposed to frequent repetition of start-and-stop. Clogging of an exhaust system with carbon or emulsion causes deterioration of combustion performance of an engine, which leads to reduction of engine power.
Therefore, it has been desired to develop a lubricating oil for two-cycle engines which can maintain their cleanliness to minimize power reduction, deterioration of starting performance and carbon clogging of their exhaust system.
As the result of extensive researches to develop a lubricating oil suitable for two-cycle engines, the present inventors have found that a lubricating oil whose base oil is a polyalkylene glycol derivative with a particular structure is much more preferable with regard to engine power, starting performance and reduction of carbon clogging of an exhaust system than commercially available, conventional lubricating oils.